AG Pipe Behind Retaining Walls: Why It Matters
Agricultural pipe, commonly known as AG pipe or aggi pipe, is a perforated or slotted flexible drainage pipe that sits behind a retaining wall at its base. Its purpose is to collect groundwater and rainwater that percolates through the soil behind the wall and channel it safely to a discharge point. AG pipe is the backbone of retaining wall drainage and is included in virtually every professionally engineered wall design.
What Is AG Pipe?
AG pipe is typically a corrugated, flexible plastic pipe available in diameters of 65mm, 90mm, or 100mm. It comes in two main types:
- Slotted AG pipe with narrow slots cut along its length that allow water to enter from the surrounding soil or gravel
- Perforated AG pipe with small holes punched at regular intervals serving the same purpose
Most AG pipe used in retaining wall applications is sold pre-wrapped in a geotextile sock (filter fabric) that prevents fine soil particles from entering and clogging the pipe. If purchasing unwrapped pipe, a separate geotextile wrap should be applied on site.
How to Install AG Pipe Behind a Retaining Wall
Correct installation is essential for the pipe to function properly. The following steps outline the standard process:
Step 1: Prepare the Trench
After the retaining wall footing is poured and the wall constructed, a trench is formed behind the wall at the base. The trench should be wide enough to accommodate the pipe plus surrounding gravel, typically 200mm to 300mm wide.
Step 2: Lay a Gravel Bed
Place a 50mm to 75mm layer of 20mm clean crushed rock in the bottom of the trench. This provides a level base for the pipe and promotes water flow toward the pipe from below.
Step 3: Position the AG Pipe
Lay the AG pipe on the gravel bed along the full length of the wall. Ensure the pipe has a minimum fall of 1 in 100 (1%) toward the discharge point. The slots or perforations should face downward so that water is collected from the gravel bed beneath.
Step 4: Cover with Gravel
Surround and cover the AG pipe with more 20mm crushed rock. The gravel backfill should extend at least 300mm behind the wall face and ideally up to two-thirds of the wall height. This gravel zone acts as a free-draining layer that channels water down to the AG pipe.
Step 5: Install Geotextile Fabric
Wrap the entire gravel zone in geotextile filter fabric to prevent the surrounding soil from migrating into the gravel and clogging the drainage layer over time. The fabric should overlap generously at joins.
Step 6: Connect to Discharge
The AG pipe must connect to a suitable discharge point. Options include connection to the stormwater system, a soakaway pit, or daylight discharge to a lower area of the property. The pipe should never terminate in a dead end.
Common AG Pipe Installation Mistakes
Even experienced DIY landscapers can make errors with AG pipe installation. Common mistakes include:
- Insufficient fall causing water to pool in the pipe rather than draining away
- No geotextile sock leading to the pipe clogging with fine sediment within a few years
- Wrong gravel using round river pebbles instead of angular crushed rock, which compacts less effectively
- Pipe too small using 65mm pipe where 100mm is needed for higher walls or wet sites
- No discharge point with the pipe simply ending in the ground
AG Pipe Sizing for Different Walls
A retaining wall engineer will specify the appropriate AG pipe diameter based on wall height, soil permeability, and expected water volume. As a general guide, 65mm pipe suits walls under 600mm, 90mm pipe is standard for walls 600mm to 1.2 metres, and 100mm pipe is recommended for walls over 1.2 metres or in areas with high water tables.
Maintaining AG Pipe Systems
Once installed behind a retaining wall, AG pipe is largely inaccessible. This makes correct initial installation critical. However, the discharge end of the pipe should be inspected periodically to ensure water flows freely and the outlet is not blocked by debris or vegetation.
Professional Installation Matters
AG pipe installation is a hidden component that is impossible to inspect or fix without excavating behind the wall. Getting it right during construction is the only practical option. We connect you with VBA-licensed retaining wall builders across Melbourne who include properly installed AG pipe drainage in every wall they construct. Find a builder today and protect your retaining wall investment from day one.
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